How many coats of undercoat?

How many coats of undercoat you need depends on surface condition, type of paint, and color change, but typically one coat of undercoat or primer is sufficient for most jobs, with two coats needed in certain situations. For previously painted walls in good condition, clean and dull, you can often skip undercoat or use one coat of undercoat if color change is drastic. For bare drywall, bare wood, or bare metal, one coat of appropriate primer undercoat is usually enough if applied evenly at recommended spread rate, as its job is to seal and bond, not to provide final color. For bare wood with knots or tannins, one coat of oil based or shellac primer may be enough, but knots may need second spot coat. For dark color to light color, one coat of stain blocking undercoat plus two topcoats is common, though some painters do two coats undercoat for better hide. For car underbody rust protection undercoating, one to two even coats is typical, with second coat for extra protection. Applying too many coats of undercoat can cause cracking and waste. So general rule is one full even coat of undercoat, with second coat only on problem areas like knots, stains, or bare patches, and then follow with two topcoats for finish. Always follow product label.